Recent molecular biology techniques permit a systematic introduction of mutations into specific genes. The success of such molecular evolution techniques depends greatly on the ability to recognize and isolate engineered cells that contain specific mutations. A considerable effort has been made to develop robots that can automatically carry out manipulations for the purpose of isolating specific rare cells, e.g., mutant cells that possess a desired phenotype. There are currently several so-called high throughput-screening systems commercially available. These systems attempt to automate repetitive manual laboratory operations for isolating specific rare cells of interest.
Isolation of these rare cells is not always possible. Often separation inaccuracies occur which result in contamination of the isolated rare cells of interest with other nonmutated or nonengineered cells. When the rare cells are cultured, the contaminating cells typically dominate the culture and proliferate more successfully than the rare cell type. The isolation procedure often results in a culture that is no more rich in rare cells than at the start of the isolation procedure.